This project focuses on field studies designed to investigate the interactions between hydrology, mineralogy,[unreadable] geology and geochemistry that result in naturally-elevated As concentrations (> 10 ug/L) in reducing[unreadable] groundwater. We will first seek to determine if a roughly linear relationship between groundwater As and[unreadable] groundwater age observed in Araihazar, Bangladesh, holds in New England. This new relationship,[unreadable] corresponding to a steady rate of As mobilization of approximately 20 ug/L per year, implies that the rate of recharge of[unreadable] an aquifer plays an important -perhaps dominant- role in regulating the spatial distribution of As in[unreadable] groundwater. Four areas in Maine and New Hampshire with known clusters of domestic wells with As[unreadable] concentrations up to 700 ug/L were selected for detailed study on the basis of bed-rock and surficial geology.[unreadable] Two new areas of Bangladesh with very high and low groundwater As concentrations, respectively, were[unreadable] selected for detailed investigation, including groundwater dating. Field observations from one site in New[unreadable] England and one site in Bangladesh will be synthesized with a reactive-transport model of a high-As plume[unreadable] along its flow path to a discharge area.[unreadable] There are striking similarities to the tectonic setting that led to the formation of the rocks that, eventually,[unreadable] produced the deposits of New England and Bangladesh where elevated groundwater As concentrations are[unreadable] observed. We propose to explore the implication of this analogy by conducting experiments designed to[unreadable] elucidate the mechanisms leading to the formation of mobilizable As in sediment. These experiments will[unreadable] include long-term laboratory and in situ incubations of model minerals (arsenopyrite and silicates), synthetic[unreadable] silicate glasses doped with arsenic, as well as rock specimens collected in the upland regions and river[unreadable] deposits from New England and Bangladesh. Mineralogical changes will be monitored by selective[unreadable] extractions, XRD, SEM-EDX, XAS-XANES and -EXAFS; microbiological changes will be tracked as well.[unreadable] We also propose to conduct push/pull experiments in the un-consolidated glacial-till aquifers tapped by the[unreadable] majority of public supply wells in New England to assess the mobility of As in response to changes in land[unreadable] use that could affect the subsurface hydrology or geochemistry.[unreadable]